(1) Field of the Invention
The present invention relates to a heat resistant polyvinyl chloride copolymer, heat and impact resistant polyvinyl chloride copolymer and a process for the preparation of these copolymers. More particularly, it relates to a heat resistant polyvinyl chloride copolymer which is excellent in heat resistance, thermal stability and processability and can be used for films, sheets, corrugated sheetings, pipes, cable ducts, deck plates, building materials and industrial components. It further relates to a heat and impact resistant copolymer which has excellent impact resistance in addition to the heat resisting properties.
(2) Description of the Prior Art
Vinyl chloride resin has been widely used because rigid, semi-rigid and flexible products can be prepared by kneading the resin with a heat stabilizer, light stabilizer, plasticizer, lubricant, filler and coloring material. Particularly for rigid products, heat resistance of molded articles has recently been required and hence vinyl chloride resin has been desired to improve heat resistance.
So-called post-chlorinated vinyl chloride resin obtained by further chlorinating common vinyl chloride resin has been known to date as a vinyl chloride resin having improved heat resistance. The post-chlorinated vinyl chloride resin, however, has inferior heat stability, is liable to discoloration and has disadvantages in terms of processability. Consequently, research for improvement of these resins has been continuing.
For example, BP 1,293,542 discloses a thermoplastic resin composition obtained by blending a vinyl chloride base polymer with a copolymer composed of N-substituted maleimide and methacrylate ester monomer containing 50% by weight or more of methyl methacrylate or with a copolymer composed of N-substituted maleimide and said monomer in combination with other copolymerizable monomers. The composition, however, is still unsatisfactory in heat resistance.
A resin obtained by copolymerizing vinyl chloride with N-cyclohexylmaleimide is known to have improved heat resistance. A method having specific preparation conditions for improving the heat resistance of this resin has been disclosed in Japanese Patent Laid-Open No. 236809/1987. In the disclosure, N-cyclohexylmaleimide is added by portions in copolymerizing with vinyl chloride. However, polyvinyl chloride copolymer obtained by the addition in portions is insufficient in thermal stability and impact resistance although heat resistance has been improved.
As set forth above, vinyl chloride resin can provide from flexible to rigid products when desired, is resistant to various chemicals and has good processability. However, further improvement in heat resistance, thermal stability and also impact resistance is required for the resin.